(1) Field of the Invention
The present invention relates to a material defect evaluation apparatus and its evaluation method, wherein a degree of deterioration of a specimen is evaluated by irradiating positron to the specimen and measuring a positron lifetime, and, especially relates to a material defect evaluation apparatus and its evaluation method, wherein a construction can be made compact and in situ non-destructive evaluation can be performed.
(2) Prior Art Statement
Positron has a weight equal to that of electron, and is a particle which has an electric charge equal to an absolute value of electric charge of electron and showing inverse positive and negative state. Positron is emitted from a radioisotope showing xcex2+ decay such as 22Na. If positron having an energy of several hundreds keV emitted from a positron source is irradiated to a specimen, positron is repeatedly crushed with electron and ion in the specimen, so that positron decays to a thermal energy level after a very short time such as 1 pico-second (1xc3x9710xe2x88x9212 second). The thus energy released positron, in the case that the specimen is metal, is annihilated with electron in a lifetime (specific to respective metals) such as about 100-300 pico-second. If there are defects such as vacant lattice points in the specimen, positron is trapped therein and is annihilated in a long lifetime specific to the defects (about 150-500 pico-second). Therefore, information regarding the defects can be obtained by measuring a lifetime in which positron is annihilated in the specimen.
As a method of measuring a positron lifetime, a sandwich method has been known. The radioisotope 22Na emits positron by xcex2+ decay. At that time, xcex3-ray having energy of 1.28 MeV is also emitted. On the other hand, when positron is annihilated with electron in the specimen, xcex3-ray having energy of 0.51 MeV is also emitted. Then, in order to measure a positron lifetime, two specimens are prepared that are made of the material whose positron lifetime is to be measured, and a ray source is sandwiched by the thus prepared two specimens, so that all the positron emitted from the ray source can be incident upon the specimens to be measured. Then, two xcex3-ray detectors are used in such a manner that one detects xcex3-ray having energy of 1.28 MeV so as to know a positron generation time and the other detects xcex3-ray having energy of 0.51 MeV so as to know a positron annihilation time, and a positron lifetime in the specimen is measured on the basis of a time difference between the positron generation time and the positron annihilation time. In this method, strictly speaking, the positron generation time is different from the positron incident time. However, if the ray source is arranged close to the specimen, it is possible to assume that the positron generation time is actually same as the positron incident time.
In the sandwich method mentioned above, it is necessary to use two specimens made of the same material. The reason is as follows. That is, if only one specimen is used, major part of positrons, whose generation time is known by detecting xcex3-ray having energy of 1.28 MeV, are annihilated in a material other than the specimen or in the atmosphere, and thus it is not possible to measure a positron lifetime of the specimen accurately. The measurement of positron lifetime can be theoretically performed in a non-destructive manner, but, at present, it is not possible to apply the measurement of positron lifetime for a non-destructive measurement of structural materials due to the limitations on measuring mentioned above. Moreover, even in a measurement on a laboratory, if the measurement of positron lifetime can be performed only by one specimen, it is possible to measure a precious specimen even though only one specimen exists in the world. In addition, even in normal specimens, it is possible to reduce a cost and save a trouble for preparing the specimens.
As a method of measuring a positron lifetime only by one specimen, there is disclosed a method wherein two positron lenses are used and positrons emitted from the ray source is converged by the two positron lenses and is incident upon the specimen (Yasuharu Shirai, et al J.Japan Inst. Metals, Vol. 59, No.6 (1995), pp. 679-680, and, Yasuharu Shirai: The production and technique, Vol. 48, No. 4 (1996), pp 50). In this method, a positron lifetime can be measured only by one specimen. However, since it is necessary to accommodate the specimen in a vacuum chamber, it is not possible to perform the measurement of positron lifetime for a large specimen. In addition, since it is necessary to use vacuum devices and electromagnetic lenses, there arises a drawback such that an apparatus is expensive.
Moreover, in the known method, in order to know the positron incident time, it is necessary to detect xcex3-ray having energy of 1.28 MeV, In addition, in order to pick-up a high-speed timing signal from xcex3-ray having high energy, it is necessary to use a high-speed scintillate and a high-speed photo-multiplier. Therefore, it is not possible to make a size of a start detector compact to a size of about 50 mm xcfx86xc3x97 about 250 mm L under without decreasing time resolution. A stop detector has also the same drawback. Since it is necessary to use two large detectors such as the start detector and the stop detector mentioned above, it is almost impossible to apply this method to a non-destructive measurement.
An object of the invention is to eliminate the drawbacks mentioned above and to provide a material defect evaluation apparatus using positron and its evaluation method wherein a non-destructive in-situ measurement of a positron lifetime of a large structure can be performed effectively in a short period of time.
According to the invention, a material defect evaluation apparatus using positron for evaluating the degree of deterioration of a specimen by measuring a positron lifetime after irradiating positron to the specimen comprises: a positron source, a positron detector and a xcex3-ray detector, wherein, the positron source, and the positron detector are arranged in a container though which a light is not transmitted, and, a positron transmitting window, through which positron emanating from the positron source and transmitting through the positron detector is transmitted outward, is arranged to the container.
Moreover, according to the invention, an evaluation method using the material defect evaluation apparatus mentioned above, comprises the steps of: detecting a pass of positron emanating from the positron source by means of the positron detector; emitting positron through the positron transmitting window to the specimen; detecting a generation of xcex3-ray due to positron annihilated in the specimen by means of the xcex3-ray detector; measuring the positron lifetime defined by an interval between the time when the pass of positron is detected by means of the positron detector and the time when the generation of xcex3-ray is detected by means of the xcex3-ray detector; and evaluating material defects of the specimen on the basis of the thus measured positron lifetime.
In the sandwich method, a positron generation time is known by detecting xcex3-ray having energy of 1.28 MeV, and this positron generation time is assumed as a time when positron is incident upon the specimen to be measured. Therefore, a ray source is not arranged apart from the specimen, and thus the ray source must be arranged close to the specimen as much as possible. As a result, the specimen is always exposed in danger of deterioration due to the ray source. Moreover, since a direction, to which xcex3-ray having energy of 1.28 MeV is emitted, is not correlated with a direction, to which positron is emitted, it is not possible to specify positron emitting direction even if xcex3-ray having energy of 1.28 MeV is detected. Therefore, it is necessary to cover the ray source by the material of specimen to be measured, so as to annihilate all the positrons in the material of specimen even if positron is emitted in any directions. If positron incident upon the specimen can be directly detected and the directly detected positron incident time can be utilized as a positron generation time without using the positron generation time known from a generation time of xcex3-ray having energy of 1.28 MeV, it is not necessary to cover the ray source by the material of specimen as is the case of the sandwich method. After the above investigation, the present inventors have found the material defect evaluation apparatus using positron and its evaluation method.